1. Field of the Invention
The present invention relates to a leather-like sheet having a good feel and good physical properties, to an aqueous resin dispersion for producing it, and to a method of using the aqueous resin dispersion for producing such a leather-like sheet.
2. Description of the Prior Art
Heretofore known in the art, leather-like sheets are fabricated, for example, by infiltrating a resin component such as urethane resin into a fibrous substrate such as nonwoven fabric followed by raising a fibrous nap on the surface of the thus-processed substrate, or by laminating a resin layer on the surface of such a substrate. For the resin component that is imparted to the nonwoven fabric, for example, known are urethane resin and acrylic resin. Urethane resin is widely used for it in view of the feel and the mechanical properties of the leather-like sheets fabricated with it.
One ordinary method for fabricating such leather-like sheets with a resin component infiltrated into the fibrous substrate thereof comprises infiltrating a resin solution in an organic solvent such as dimethylformamide into a fibrous substrate such as nonwoven fabric, followed by solidifying the resin in the substrate. However, the method that uses an organic solvent is unfavorable from the viewpoint of the environmental protection and the process safety. Taking these into consideration, various methods that are substitutable for the method with an organic solvent have been proposed. They comprise impregnating a fibrous substrate with an aqueous resin dispersion to fabricate leather-like sheets, but most of them are not as yet industrialized. One reason is, when an aqueous resin dispersion is used in fabricating leather-like sheets, the resin may locally move inside the fibrous substrate during the step of drying the aqueous resin dispersion, thereby giving a structural morphology with fibers strongly restrained by the locally-concentrated resin, and, as a result, the sheets fabricated may lose their flexibility and have a stiff feel.
To solve the problem with the process that uses an aqueous resin dispersion, various methods have been proposed for uniformly introducing resin into a fibrous substrate. For example, after an aqueous resin dispersion has been infiltrated into a fibrous substrate, it is heat-sensitive gelled. Concretely, one method proposed for fabricating artificial leather comprises applying an emulsion that has been prepared by dissolving an inorganic salt in a nonionic surfactant-containing aqueous polyurethane emulsion, to a fibrous substrate followed by drying it under heat (JP-A 6-316877, 7-229071). However, the artificial leather fabricated according to the method proposed in these patent references is still unsatisfactory in point of the flexibility, fulfillment, feel and durability thereof. This is because the resin concentration in the emulsion used in the proposed method is at most about 10% and is extremely low and therefore only a small amount of the resin may be infiltrated into the fibrous substrate, and, in addition, when the resin concentration in the emulsion is not larger than 20%, the resin does not almost heat-sensitive gel. On the other hand, if a large amount of an inorganic salt is added to the emulsion and when the resin concentration in the emulsion is high, for example, 30% or more, then the emulsion may thicken or gel by itself and may be unstable with elapsed time.
Another method proposed for fabricating leather-like sheets comprises infiltrating an urethane resin emulsion or an urethane-acrylic composite resin emulsion having a heat sensitive gellability in which the resin has an elastic modulus falling within a specific range, into a fibrous substrate followed by heat-sensitive gelling it and drying it (JP-2000-17582, 2000-303370). The leather like-sheets fabricated therein are surely improved in point of the feel and the physical properties thereof, but the method proposed in these patent references, which comprises applying such a heat-sensitive gellable resin to a fibrous substrate, is still problematic and will be industrially after all impracticable since the emulsion to be used therein may thicken with elapsed time and may gel within a few hours to a few days even at around room temperature. In addition, if a low-molecular surfactant such as that shown in the patent references is used as a heat-sensitive gelling agent, then the feel and the physical properties of the leather-like sheets obtained may lower. This is because, when the sheets are dyed or are processed with hot water for converting the fibers of the fibrous substrate into ultra-fine fibers, then they may be significantly deformed owing to the water absorption by the resin therein.
Still another method proposed for fabricating artificial leather comprises infiltrating an aqueous resin composition that comprises an aqueous urethane resin, an inorganic salt, and a nonionic surfactant having a clouding point, into a fibrous substrate, followed by heat-sensitive gelling it and drying it (JP-A 11-335975). Even in this method, however, it is still difficult to smoothly fabricate leather-like sheets since the emulsion used therein is unstable and, especially in summer, the emulsion may thicken or gel by itself.
On the other hand, a method for producing an emulsion that is stable in a liquid and has a heat-sensitive gellability has been proposed, which comprises adding an organopolysiloxane-type heat-sensitive gelling agent or a nonionic surfactant-type heat-sensitive gelling agent is added to an emulsion of an acrylic resin having a specific functional group (JP-A 6-256617, 6-329867, 7-90154). In the method, however, the structure of the resin to be used is limited to an extremely narrow range, and the resin of the type is unfavorable for fabrication of leather-like sheets. These patent references are silent at all on a method of making urethane resin and urethane-acrylic composite resin that are especially favorable for fabrication of leather like-sheets, heat-sensitive gellable. In these patent references, described are organopolysiloxane and alkylphenol-formaldehyde condensate alkylene oxide adduct that are favorable for the heat-sensitive gelling agent. However, the former is extremely expensive and is therefore industrially disadvantageous since it increases the product costs; and the latter is unfavorable since its raw material, alkylphenol is a type of hormone disrupting chemicals, and formaldehyde causes sick house syndromes. Further proposed is an aqueous resin composition of which the viscosity reversibly increases relative to the ambient temperature (JP-A 9-111133). However, the aqueous resin composition described in this patent reference does not irreversibly gel but its viscosity reversibly increases, and even when the aqueous resin composition of the type is used in fabricating leather-like sheets, it may again liquefy while it is heat-sensitive gelled and dried, and the resin may move in the dried fibrous substrate. If so, the resin having been infiltrated into the substrate may have a structural morphology with fibers strongly restrained by the locally-concentrated resin, and, as a result, the sheets fabricated may lose their flexibility and have a stiff feel. In our further studies, we, the present inventors have found that the viscosity-increasing compounds shown in this patent reference are not so effective for heat-sensitive gelling of surfactant-stabilized urethane resin and urethane-acrylic composite resin that are favorable for fabrication of leather-like sheets, and therefore the compounds are unfavorable for fabrication of leather-like sheets.